Get tables and views

[kbarbounakis]

This PR implements two extra helpers for getting tables and views as list. PostgreSQLAdapter.tables() returns a helper for getting the collection of tables:

interface DataAdapterTables {
    list(callback: (err: Error, result: { name: string, owner?: string, schema?: string }[]) => void): void;
    listAsync(): Promise<{ name: string, owner?: string, schema?: string }[]>;
}

and PostgreSQLAdapter.views() returns a helper for getting the collection of views:

interface DataAdapterViews {
    list(callback: (err: Error, result: { name: string, owner?: string, schema?: string }[]) => void): void;
    listAsync(): Promise<{ name: string, owner?: string, schema?: string }[]>;
}